As the requirement for straight-line interconnection of pipes is widespread, a large market exists for pipe couplings for this purpose, and thousands of different designs have been disclosed. Some have been commercially developed as a solution for closely-defined requirements. In choosing a coupling much depends on factors such as the material of the pipe, whether an external anchor (such as a flange) is available, pipe diameter and variation of said diameter expected, the fluid being transported, stresses expected, environmental conditions, whether the joint is to be permanent or openable, whether welding is allowed or not, the allowed hydraulic resistance and further factors. Thus, there is no best valve for all applications. All that can be said is that the best valve is the lowest cost valve that meets all the requirements of a specified application.
Many satisfactory solutions exist and are in use for the joining of small diameter pipes made of copper, steel, plastics, aluminium and other materials. While band clamp couplings can be used for diameters as small as 25 mm, usually other well-established methods are more appropriate and economical. The present invention is thus concerned primarily with pipe diameters over 75 mm (about 3″).
The following U.S. patents provide an indication of the state of the art in this field.
In U.S. Pat. No. 3,877,733 the present inventor disclosed a pipe coupling with a sealing gasket. The coupling was satisfactory for applications where no substantial axial forces need to be resisted. The present inventor disclosed similar though more complex couplings including a spring element in U.S. Pat. Nos. 4,119,333 and 4,629,217.
In U.S. Pat. No. 6,502,865 Steele discloses a peripherally-clamped coupling which does not intrude on the pipe inner diameter. The first embodiment of the coupler does however require the welding on of a square-section ring on the outside of both pipes. In many applications welding is time-consuming, prohibited or impossible, which limits the applications of this type of coupling.
A deformable metal sleeve is used as a pipe connector in the proposed coupling disclosed by Nghiem in U.S. Pat. No. 6,581,982. Axial force is applied to the sleeve which then distorts and compression-enters soft material in the coupling flange. This needs to be repeated for the second pipe and for the second coupling flange. Application of a high axial force in the field is difficult, and there is no way either to further tighten or to open the coupling. The same remarks apply to the coupling disclosed by Readman in U.S. Pat. No. 6,595,559, which has an additional disadvantage in that the flow of fluid in the pipe is somewhat restricted.
A pipe clamp composed of two clamp halves, each provided with its own tension band is disclosed by Weinhold in U.S. Pat. No. 6,672,631. The device has a quick-acting closure, but no protection against axial separating forces, and in use it is difficult to prevent fluid leakage due to gaps between the two bands.
Wraith et al disclose a plastic pipe coupling in U.S. Pat. No. 6,676,166. The device would not be usable for large diameter steel pipes as the assembly forces required would be very large and also the coupling can not be tightened to prevent leakage.
The design seen in U.S. Pat. No. 6,739,630 to Riedy is unsuitable for large diameter pipes as it requires one end of one of the pipes to carry an external threw thread. Furthermore there remains the difficult task of revolving the large diameter nut sleeve while preventing revolution of the pipe carrying the male thread in order to apply the coupling.
A further band clamp coupling is seen in U.S. Pat. No. 6,749,232 to Wachter et al. Provision is made for deterring axial separation, but sealing this type of clamp against leakage is problematic due to the gap between the ends of the band.
Komolrochanaporn discloses a push-pull pipe coupling in U.S. Pat. No. 6,824,172. Both fluid sealing and mechanical retention are provided. There is however no way of tightening the seal and this type of coupler is unsuited for large diameter pipes and cannot handle pipe diameter variations.
Rex et al. disclose a pipe coupling in U.S. Pat. No. 6,843,514 which could be suitable for large diameter pipes. It is however not clear how the gripping device is to handle pipe diameter variations. The thin seal used and its indirect manner of applying pressure to the outer surface of the pipes appear to be the weak points of this design.
In U.S. Pat. No. 6,851,728 Minami also discloses a pipe coupling provided with both sealing and gripping members. However the design does not make provision for substantial outer diameter variations and the locking mechanism would be difficult to operate if applied to large diameter pipes.
Wartluft in U.S. Pat. No. 6,877,777 discloses a complex coupling which requires the insertion of a sleeve into the bore of the pipes to be interconnected. Any intrusion into the inner space of a pipe is of course detrimental in increasing the hydraulic resistance of the pipeline.
One of the shortcomings of prior-art pipe couplings is an inability to interconnect pipes varying more than about 3 mm from the nominal outside diameter. Variations larger than this can occur, either through manufacturing variations or because the pipe has been coated with some corrosion-protection material.